1. Field of the Invention
The present invention relates to an ink jet recording apparatus, and more particularly to an ink jet recording apparatus having a paper gap adjusting mechanism for adjusting the distance between a recording head and a platen, corresponding to the kind of recording medium or the recording pattern.
2. Related Background Art
Conventionally, recording apparatuses have been provided with a paper gap adjusting mechanism for adjusting the gap between a recording head and a recording sheet to enable the recording of invariable quality for recording sheets having different thicknesses.
FIG. 1 is a side view showing a constitutional example of a recording unit for an ink jet recording apparatus as an example which has a paper gap adjusting mechanism for adjusting the gap between the recording sheet and the ink jet recording head by rotating the head.
A sheet 11 supplied from a sheet supply tray 15 by the operation of a sheet supply roller 13 and a separation pad 14 is detected by a sheet top detection lever 8, and then conveyed a fixed amount to come under the recording head 4.
A carriage shaft 3 and a pinch roller base 12 are supported by left and right plates, not shown, in the apparatus, a carriage 2 being engaged freely rotatably around the carriage shaft 3 via a carriage bearing 2D.
The paper gap adjusting lever 1 is rotatably attached to the carriage 2 via a shaft 1A. A stepped cam 1B attached near an engagement portion of the paper gap adjusting lever 1 around the shaft 1A is pressed against the pinch roller base 12 by the dead weight of the carriage 2 and the paper gap adjusting lever 1. A projection 1C provided near a halfway point of the lever 1 can be engaged into any of three concave portions formed on a side face of the carriage 2 by the rotation of the lever 1, thereby occupying one of a first position 2A, a second position 2B and a third position 2C of the lever 1. With the above position of this lever 1 varied, a respective one of the faces of the stepped cam 1B which have different heights abuts on the pinch roller base 12, whereby the distance between an orifice face of the head 4 secured to the carriage 2 and the platen can be changed in three ways.
The carriage 2 is reciprocated in the sheet width direction along the carriage shaft 3 and the pinch roller base 12, to make the recording onto the recording sheet 11 by discharging the ink from the recording head 4, along with the movement of the carriage 2. A pinch roller 7 is biased by a spring, thereby forcing the recording sheet 11 onto a conveying roller 6.
FIGS. 2 to 4 are side views showing the paper gap distance when the paper gap adjusting lever 1 is placed at the first, second and third positions, respectively. As will be clear from the views, the paper gap distance increases in the order of the first, second and third positions, such as 1.2 mm, 1.7 mm and 2.2 mm.
Reference is further made to FIG. 5. By rotating or turning the recording head in a Y direction around a virtual rotational center 3A of the recording head 4, the paper gap distance between the recording sheet 11 and the recording head 4 can be adjusted. In this regard, if the paper gap distance is varied from h1 to h2 by turning the recording head 4, ink droplets discharged from the ink jet recording head 4 onto the recording sheet 11 occur in f1 and f2 directions to impinge at 1.sub.1 and 1.sub.2, respectively. Note that the Z direction is a conveying direction of the recording sheet 11.
However, in the conventional example as above described, the search position of the head of the recording sheet, that is, the conveying amount x (see FIGS. 2 to 4 and when the recording sheets 11 are conveyed in a rightwise direction in FIG. 5) after the top of the sheet is detected by the sheet top detection lever 8 is always constant, irrespective of the distance between the recording head and the platen (paper gap distance). Therefore, there was a problem that if the paper gap adjustment is made corresponding to the kind of the recording sheet or the recording pattern, the start position of recording, that is, the position at which the ink is first discharged from the recording head 4 onto the top portion of the recording sheet 1, may be different depending on the paper gap distance, such as 3.5 mm, 3.1 mm and 2.8 mm, as shown in FIGS. 2 to 4. That is, there was a problem that an offset .DELTA.1 of the ink impinging or reaching position may occur as shown in FIG. 5.
Also, when the adjusting width of the paper gap distance according to the position of the lever 1 is greater, e.g., when the lever 1 is shifted to the third position, there is the possibility that as the top of the recording sheet 11 does not already arrive to the discharge position of the recording head 4, the recording may occur outside of the recording sheet. If the attempt is made to prevent this, there was a problem that the margin at the top of the sheet is excessively large in recording at the first position, resulting in the recordable range being narrowed.
Also, in an arrangement that the ink discharge orifices are disposed ahead of the rotation center of the recording head in the conveying direction of the recording sheet 11 and that the recording sheet 11 is conveyed in a leftwise direction in FIG. 5, if the paper gap distance is large, there was a problem that the ink may impinge beyond the end of the sheet.
Furthermore, the above problems may likewise arise in the constitution in which the recording head 4 is oscillated around the center of oscillation with respect to the recording sheet 11.